The speeds of metal-oxide-semiconductor (MOS) transistors are closely related to the drive currents of the MOS transistors, which drive currents are further closely related to the mobility of charges. For example, NMOS transistors have high drive currents when the electron mobility in their channel regions is high, while PMOS transistors have high drive currents when the hole mobility in their channel regions is high.
Compound semiconductor materials of group III and group V elements (referred to as III-V compound semiconductors hereinafter) are good candidates for forming NMOS devices due to their high electron mobility. In addition, germanium is a commonly known semiconductor material, and the electron mobility and hole mobility of germanium are greater than that of silicon, which is the most commonly used semiconductor material in the formation of integrated circuits. Hence, germanium is also an excellent material for forming integrated circuits. Therefore, III-V based and germanium-based transistors have been recently explored.
A challenge faced by the semiconductor industry, however, is that although the MOS transistors formed on III-V compound semiconductors or germanium have high drive currents, the leakage currents of these MOS transistors are also high. This is partially caused by the low bandgap and the high dielectric constants of the III-V compound semiconductors and germanium. For example, FIG. 1 illustrates the comparison of bandgaps and dielectric constants of germanium, commonly used III-V compound semiconductors, and other semiconductor materials such as group IV materials. FIG. 1 reveals that the bandgaps of germanium and some commonly used III-V compound semiconductors are small. Therefore, the respective MOS transistors suffer from high band-to-band leakage currents between their gates and source/drain regions. The high dielectric constants of these materials further worsen the leakage currents. As a result, the on/off current ratios (Ion/Ioff) of III-V based MOS transistors and germanium-based MOS transistors are relatively low.